Aug 31 / Iakovos Koukas

The Normal Distribution or Bell Curve

The normal (or Gaussian) distribution of IQ scores, also known as the Bell Curve, is a bell-shaped curve. It is assumed that all IQ score measurements across the overall population will follow a normal probability distribution and have an equal number of measurements before and after the mean value.

The method for all IQ tests is the "deviation IQ." Deviation IQ is calculated as a function of the discrepancy of an individual score from the mean score. An individual's raw 13 score on a given test is transformed to a normal (or Gaussian) distribution, also known as the Bell Curve, which is symmetric around its mean. The mean (or average) is 100, and the standard deviation is 15 (used in most professional IQ tests). An IQ score of 100 indicates that the test-takers cognitive performance on the given test is at the mean or median level of performance in the statistical sample of test-takers of about the same age who were used to norm the IQ test. An IQ score of 115 shows a cognitive performance of one standard deviation above the median of 100, a score of 85 performance, one standard deviation below the median, and so on.

68% of the population scores between 85 and 115, 95% scores between 70 and 130, 99% scores between 55 and 145, and only 2% scores below 70 or above 130.